Effects of molecular weight of grafted hyaluronic acid on wear initiation.

Hyaluronic acid (HA) of different molecular weights (Mw) was grafted onto mica surfaces to study the effects of Mw on the conformation and wear protection properties of a grafted HA (gHA) layer in lubricin (LUB) and bovine synovial fluid (BSF) using a surface forces apparatus. The Mw of gHA had significant effects on the wear pressure (Pw), at which point the wear initiates. Increasing the gHA Mw from 51 to 2590kDa increased Pw from 4 to 8MPa in LUB and from 15 to 31MPa in BSF. The 2590kDa gHA in BSF had the best wear protection (Pw∼31MPa), even though it exhibited the highest friction coefficient (μ∼0.35), indicating that a low μ does not necessarily result in good wear protection, as is often assumed. The normal force profile indicated that BSF confines the gHA structure, making it polymer brush-like, commonly considered as an excellent structure for boundary lubrication.